This invention relates to glass compositions having a high heat stress factor. More particularly, the present invention concerns thermally high pre-stressable strength and consequently high mechanical strength-glasses (high .sigma..sub.B B -values), simultaneously featuring low heat expansion coefficients due to high temperature shock resistance or, in other words featuring high hot stressing factors.
When expressing the serviceability of partly heated glass objects, the heat stress factor as outlined below ##EQU1## (please see "Die Keramik", by Salmang-Scholze, Springer Verlag, Berlin, 1968, p. 334 et seq.), is more informative than the separate evaluation of .sigma..sub.B and .alpha.. The greater is the value for R, the higher the resistance against partial heating.
Within this formula, the symbols have the following meaning:
.sigma..sub.B =bending tensile strength (kp/cm.sup.2) with undamaged surface or surface broached with wet 220 grain emery. In the latter case, the values are approximately 400 kp/cm.sup.2 lower than the values obtained on undamaged test-piece surfaces. PA1 .mu.=transverse contraction index--approximately 0.20 for silicate glasses PA1 .alpha.=mean linear thermal expansion coefficient in the temperature range of 20.degree.-300.degree. C. PA1 E=elasticity modulus (kp/cm.sup.2): between approximately 6.5 and 9.0.times.10.sup.5 kp/cm.sup.2 for silicate glasses. PA1 1. .alpha.'/.alpha..gtoreq.4.0; i.e., to achieve a high compression prestressing (high .sigma..sub.B) the ratio of thermal expansion coefficent .alpha.' above transformation temperature (Tg) to the thermal expansion coefficient below Tg is decisive. .alpha.-values at 20.degree.-300.degree. C. (viz. below Tg) do not suffice alone to obtaind an optimum compression prestressing . 2. Softening point of glasses Ew (.rho.=10.sup.7.6 poise)&gt;820.degree.. PA1 4. Ew-Tg=232.degree.-298.degree. C.
Of these four values determining the heat stress factor R, .sigma..sub.B and .alpha. are of particular importance. Under approximately constant .mu.-values and limited differences in E-modulus for the glasses according to the invention, R is determined practically alone by those values. It follows from this that temperature shock-resistant glasses with high R-values are only obtained where .alpha. is low and .sigma..sub.B is high. Practice shows that this aim has so far only been achieved with low heat expansion glasses even where, as in the case of borosilicate glass with an .alpha./300 value of approximately 33.times.10.sup.-7 /.degree.C., the glasses allow a less pronounced thermal pre-stress (pressure stress produced in the glass surface) than glasses with a high thermal expansion, such as window glass (float glass) with an .alpha./300-value of approximately 90.times.10.sup.-7 /.degree.C.
It must be recalled here that glasses with a high thermal expansion do not lend themselves easily to high pressure pre-stressing. The limit results from the fact that the equivalent tensile stress, automatically created within the glass during pre-stressing, must not exceed the basic strength of the glass as predetermined by internal defects such as bubbles, waviness and the like, since a fracture will otherwise occur. (Whenever glass breaks, this is invariably because of excessive tensile stresses, since the compression strength of glass is some 10 times greater than the tensile strength).
The specific pre-stressability (hardenability) is also dependent upon the linear thermal expansion coefficient of the glass; the surface compression stress achieved is approx. 400 kp/cm.sup.2 for the above borosilicate glass and approx. 1200 kp/cm.sup.2 for window glass, as determined upon 50.times.20.times.5 mm laboratory samples.